1. Field of the Invention
This invention relates to a switching power supply device having a main switching element and a series capacitor in an input line thereof.
2. Description of the Related Art
A circuit drawing of a conventional switching power supply device is shown in FIG. 2. The switching power supply device comprises a main switching element Q1, and is configured as having a switching power supply circuit 1 in which its primary and secondary sides are insulated therebetween by a transformer T1, and a pulse oscillation circuit IC1 for outputting a pulse signal in order to control timing of switching of the main switching element Q1.
To one end of the primary winding of the transformer T1 owned by the switching power supply circuit 1, the drain of the main switching element Q1 composed of an FET is connected in series. The other end of the primary winding of the transformer T1 is connected to one pole of DC terminals of a first rectifier bridge D1, and a series capacitor C4 is connected to one pole of AC terminals of the first rectifier bridge D1. To the series capacitor C4, a photo-coupler PC1 is connected in parallel. One pole of DC terminals of the first rectifier bridge D1 is connected to one end of the primary winding of the transformer T1, and the other pole of the DC terminals of the first rectifier bridge D1 is connected to the source of the main switching element Q1. The drain of the main switching element Q1 is connected to the other end of the primary winding of the transformer T1. Between the two DC terminals of the first rectifier bridge D1, a smoothing capacitor C8 is connected so as to allow itself to serve as a DC power supply based on the rectifying action of the first rectifier bridge D1.
A second series capacitor C3 is connected to the series capacitor C4 on the load side thereof, and the series capacitor C3 is connected to one pole of AC terminals of a second rectifier bridge D2. The other pole of the AC terminals of the second rectifier bridge D2 is connected via a capacitor C6 to the other pole of an AC power source. Between two DC terminals of the second rectifier bridge D2, a smoothing capacitor C11 is connected so as to allow itself to serve as a DC power source based on the rectifying action of the second rectifier bridge D2.
To the DC terminals of the second rectifier bridge D2, a pulse oscillation circuit IC1 is connected. More specifically, one pole of the second rectifier bridge D2 is connected to a power source input terminal VCC of the pulse oscillation circuit IC1, and the other pole of the second rectifier bridge D2 is connected to an RT/CT terminal of the pulse oscillation circuit IC1, respectively. An output terminal OUT of the pulse oscillation circuit IC1 is connected to the gate of the main switching element Q1, and a current detection terminal ISNF of the pulse oscillation circuit IC1 is connected to the source of the main switching element Q1 (Refer to Japanese Published Unexamined Application published by official bulletin No. Hei 10-14227 (see FIG. 1 and Page 3 to 5), etc.).
The pulse oscillation circuit IC1 is connected to a switching element Q4. To the input terminal of the switching element Q4, a stand-by signal sender circuit 5 for sending a stand-by signal from an external device to the switching power supply device is connected. To the output terminal of the switching element Q4, a photo-coupler PC3 is connected, so as to allow a signal received by the photo-coupler PC3 to be further received by the photo-coupler PC1 provided on the input line thereby controlling ON/OFF timing of the main switching element Q1.
The above-described configuration requires the stand-by signal for turning on/off the main switching element Q1, and it was therefore necessary to separately provide the stand-by signal sender circuit 5 which receives the stand-by signal from an external device and sends it to the main switching element Q1. It was still also necessary to match an interface with the external device in order to send or receive the signal, and this has failed in establishing a standard specification and generality.